How well can aquarium fish see?

Much depends on the natural environment where they occur. Among those
species with the most developed eyesight is the so-called four-eyed
fish (Anableps anableps) which is found in parts of Central and South
America. These fish inhabit quite shallow, clear water and spend much
of their time close to the surface.
Each eye is actually divided
into two parts, with one part being developed to see well in air, while
the other operates underwater. Each segment of the eye operates
independently, so they can scour the water looking for possible food,
while also being on the look-out for possible danger from above.

By
way of contrast, the blind cave fish (Astyanax fasciatus mexicanus) has
no vision at all, as its name suggests. Although these fish actually
hatch with functional eyes, skin soon grows over them, so they lose
their sight at an early age. This is because vision is of no value in
the dark Mexican caves which they inhabit - they have also lost their
body pigmentation, and so appear white.

Blind cave fish have become isolated here and have
adapted to this new environment, whereas their normally-sighted and
colored relatives are to be found in nearby rivers. (Photo shown here courtesy of JohnstonDJ).